The present invention relates generally to an adjustable speed drive system for an induction motor and more particularly, to a vector control system having compensation for a secondary resistance variation.
In the vector control system using the slip frequency control method, a secondary resistance of the induction motor is used for calculating a slip frequency, so that a variation in temperature of the secondary resistance causes a deterioration of torque control characteristic.
Some compensation methods of a variation of the secondary resistance are proposed. One is such that compensation for a secondary resistance variation is carried out by determining constants of the induction motor, and considering a difference between an output voltage of a model of the induction motor using these constants and an actual output voltage to be a variation due to a variation in temperature of the secondary constants.
With this method, however, in connection with a primary voltage, a command value due to a dead time of an inverter or a voltage drop of main circuit elements is sometimes different from an actual value, so that there is a limit with respect to achievement of high torque control accuracy. Moreover, this method cannot correspond to a change in temperature of a primary resistance.
Another method is such that compensation for a secondary resistance variation is carried out based on the fact that on the .gamma.-.delta. axes having a primary current set as a reference axis, a voltage component of the .delta. axis which is normal to the primary current is not influenced by the primary resistance.
This method enables compensation for a secondary resistance variation which is robust to a change of the primary resistance, since a current control system is constructed on the d-q axes having a secondary magnetic flux set as a reference axis, and a component of a primary voltage variation on the .delta. axis is detected by a pulse width modulation (PWM) signal. For further information, see, for example, Paper D, vol. 112, No. 2, pp. 107-116, published in 1993 by Electric Society, or JP-A 3-253288.
With the latter method, however, due to a power source voltage, etc., the adjustable speed drive system has an upper limit of a possible output voltage, so that when a current control amplifier (refer hereinafter to as ACR amplifier) outputs a voltage command above the upper limit (which is generally called voltage saturation), an error is produced between a current command and a current passing through the induction motor. In view of this error, the latter method is available only in an area without voltage saturation. Further, when a voltage type PWM inverter is used in a voltage control part, a pulse lack is produced when the PWM pulse width becomes smaller than the dead time, resulting in a deterioration of current accuracy in the amplitude and phase. In this case, compensation for a secondary resistance variation has an error.
Moreover, although the latter method enables compensation for a secondary resistance variation which is robust to a change of the primary resistance, if set values of an exciting inductance M' (=M.sup.2 /L.sub.2 wherein L.sub.2 is a secondary inductance) and an equivalent leakage inductance L.sigma. have an error, this method suffers an influence of a voltage error.
It is, therefore, an object of the present invention to provide a vector control system for an induction motor which enables more accurate compensation for a secondary resistance variation.